1. Field of the Invention
This invention relates to a charging roller, a process cartridge and an electrophotographic apparatus. More particularly, it relates to a charging roller which is a conductive member provided in contact with an electrophotographic photosensitive member and is used in a contact charging assembly which charges the electrophotographic photosensitive member electrostatically to a stated potential upon application of a voltage; and a process cartridge and an electrophotographic apparatus which make use of such a charging roller.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. Copies are commonly obtained by forming an electrostatic latent image on an electrophotographic photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image (a toner image), transferring the toner image to a transfer medium such as paper, and thereafter fixing the toner image to the transfer medium by heat and/or pressure. The toner not transferred to the transfer medium and remaining on the photosensitive member is also removed through a cleaning step from the surface of the photosensitive member.
As charging assemblies for electrophotography, corona charging assemblies have conventionally been used. In recent years, contact charging assemblies have been put into practical use in place of the former. This aims at low ozone and low power consumption. In particular, a roller charging system making use of a charging roller as a charging member is preferably used in view of the stability of charging.
In the roller charging, a conductive elastic roller is brought into pressure contact with a member to be charged (charging object) and a voltage is applied thereto to charge the charging object electrostatically.
Stated specifically, the charging is performed by causing electric discharge from the charging member to the charging object, and hence the charging takes place upon application of a voltage above a certain threshold voltage. For example, when a charging roller is brought into pressure contact with an organic electrophotographic photosensitive member having a 25 xcexcm thick photosensitive layer and containing an organic photoconductive material (OPC electrophotographic photosensitive member), the surface potential of the electrophotographic photosensitive member begins to rise upon application of a voltage of about 640 V or above as absolute value and, at voltages above that voltage, the electrophotographic photosensitive member surface potential linearly increases at a gradient 1 with respect to the applied voltage. This threshold voltage is hereinafter defined as charging start voltage Vth.
That is, in order to attain an electrophotographic photosensitive member""s surface potential Vd considered necessary for electrophotography, a DC voltage of Vd+Vth that is higher than is necessary must be applied to the charging roller. Such a method of applying only DC voltage to the contact charging member in this way is called DC charging.
In DC charging, however, it has been difficult to control the potential of the electrophotographic photosensitive member to the desired value, because any environmental variations may cause variations in electrical resistance of the contact charging member and also because any changes in layer thickness due to scrape of the electrophotographic photosensitive member surface may cause variations in the Vth.
Accordingly, in order to achieve more uniform charging, as disclosed in Japanese Patent Application Laid-open No. 63-149669, an AC+DC charging system is used in which a voltage formed by superimposing an AC component with a 2xc3x97Vth or higher peak-to-peak voltage on a DC component corresponding to the desired Vd. This is a system aiming at a potential-leveling effect which is attributable to AC, where the potential of the charging object converges to the Vd, the middle of the peak of AC voltage, and may hardly be affected by external disturbance such as environmental variations.
Meanwhile, as the conductive member for charging, U.S. Pat. No. 4,967,231 discloses an example in which using a conductive seamless tube a surface layer is formed on a conductive support member. Japanese Patent Application Laid-open No. 5-2313 also discloses a seamless tube comprised of a fluorine resin, and Japanese Patent Application Laid-open No. 5-96648 discloses a multi-layer tube comprised of layer construction having different conductivities. As a method concerning how to manufacture them as charging members, a method is available in which the tube is fitted onto the support member to form the surface layer. Japanese Patent Application Laid-open No. 6-58325 also discloses a method of forming the surface layer by means of a crosshead extruder.
Such methods of forming the charging roller by using the seamless tube enables formation of smooth surface even when a foam is used as an elastic layer formed on a substrate, because it may further be covered with the seamless tube. Thus, more uniform charging can be performed with ease.
However, where a seamless tube formed using a thermoplastic resin as a base is used, it has been difficult to provide a charging roller which can attain uniform charging performance and reproduced-image quality which are stable and good, because, e.g., changes in electrical resistance tend to occur for the expansion and contraction of roller caused by any changes in environment and changes with time, and any defects such as surface scratches tend to affect images.
An object of the present invention is to provide a charging roller which can make changes in electrical resistance small against the expansion and contraction of roller caused by any changes in environment and changes with time, and also can make it hard for any defects such as surface scratches to affect images and can promise uniform charging performance and reproduced-image quality which are stable and good.
Another object of the present invention is to provide a process cartridge and an electrophotographic apparatus which have such a charging roller.
That is, the present invention is a charging roller comprising a support member and a conductive cover layer;
the conductive cover layer being formed of a seamless tube, and the seamless tube containing at least the following components (A), (B) and (C):
(A) a thermoplastic styrene elastomer;
(B) a polystyrene resin; and
(C) a carbon black;
and the proportion of the component (A) to the component (B) being (A)/(B)=60/40 to 20/80 in weight ratio.
The present invention is also a process cartridge and an electrophotographic apparatus which have the above charging roller.